Usual place or you could try calling a local GB reco shop. https://conceptzperformance.com/search.php?search=baulk+300zx
Synchro parts Having replaced all the synchros in a box I would suggest you beware that there are quite a few service files on these parts. I think you will be OK with doing 1 through 4 - third is one of the more chunky double cone synchros. I ended up without change from about $900 in parts (when AUD/USD was a lot better) to re-do all the synchros in my last box. The fifth gear/reverse synchros are different on newer transmission (as are the shafts and couplings and bearings and all these need replacing when you do these due to the old parts being unavailable and under Service Files).
Thanks Ltd. I am only replacing the 3rd gear syncro atm. I ended up getting it straight from our local Nissan dealer at what id say a reasonable price. $136 I was supprised, as every other time i have contacted them in regard to Zed spare parts, i have had not 1 bit of luck at all. This time around, i had someone who was actually willing to help. Always makes things easier. Col